Epitaxial structure of gallium nitride series semiconductor device and process of manufacturing the same

ABSTRACT

An epitaxial stricture of a gallium nitride series semiconductor device and a process of forming the same are described. A first buffer layer of gallium nitride is epitaxially formed on a substrate at a first temperature. A second buffer layer of indium gallium nitride is formed on the first buffer layer at a second temperature. The second temperature increases up to a third temperature, during which precursors including In(CH 3 ) 3  and NH 3  are used for surface treatment. A high-temperature gallium nitride is formed at the third temperature. The buffer layer and the way to form such a buffer layer allow improved crystal configuration and lowered defect density, thereby increasing the performance and service life of a semiconductor device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to an epitaxial structure of a galliumnitride semiconductor device and a process of manufacturing the same,and more particularly to a process of growing an epitaxial layer as abuffer layer.

2. Description of the Related Art

Illumination devices can be formed of different layers. In the case thatthe layers are epitaxially formed, defects are inevitably generated intheir crystal structures. Performance of the illumination devices isadversely affected thereby in several aspects such as, for example,reduced illumination efficiency, lowered electron shifting, andprolonged diffusion paths for dopants. Moreover, V-shaped trenches,resulting in layer dislocation, appear in the quantum wells of activelayers. Furthermore, initial reverse bias is also increased. If cracksor gaps appear in the crystal structure, an illumination device cannotbe grown above the cracks or gaps, because the illumination deviceformed on such an area has a short service life and low illuminationefficiency. Therefore, how to form an epitaxial layer with a perfectcrystal structure is the key to, improve the performance of theillumination device.

Gallium nitride series material can be used as a wide-bandgapsemiconductor device that emits various lights, from green to violet.Gallium nitride bulks are difficult to grow so that, currently, galliumnitride must be formed on a sapphire- or SiC-based substrate. Latticeconstant of the substrate is not consistent with that of galliumnitride. The gallium nitride layer directly formed on the substrate isnot reliable and therefore a buffer layer must be formed between thesubstrate and the gallium nitride. The buffer layer is also called anucleation layer with a lattice constant similar to that of thesubstrate. Nucleation and growth of gallium nitride are performed on thebuffer layer to form a crystal structure significantly the same as thesubstrate, thereby increasing the crystallization of the galliumnitride-series layer. Therefore, the quality of the buffer layer greatlyinfluences expitaxy of a cladding layer and an active layer subsequentlyformed, and indirectly influences properties of the illumination device.

U.S. Pat. No. 5,290,393 discloses a gallium nitride grown on asapphire-based substrate, as shown in FIG. 1. A low-temperature aluminumnitride buffer layer 12 of 0.001-0.5 μm in thickness is formed on thesapphire-based substrate 11. A high-temperature aluminum gallium nitridebuffer layer 13 is formed on the low-temperature aluminum nitride bufferlayer 12. Generally, the temperature at which the low-temperaturealuminum nitride buffer layer 12 is grown ranges from 200° C. to 900° C.The temperature at which the high-temperature aluminum gallium nitridebuffer layer 13 is grown ranges from 900° C. to 1150° C. This methodincreases the crystallization of gallium nitride series compound.However, the defect density of 4cm-thick aluminum gallium nitride layer13 is still as high as 10⁹-10¹⁰ cm⁻². U.S. Pat. No. 6,252,261 uses anELOG method to reduce the defect density, as shown in FIG. 2. A baselayer 22 of gallium nitride is epitaxially grown by MOCVD on asapphire-based substrate 21. The base layer 22 includes alow-temperature gallium nitride buffer layer and a high-temperaturegallium nitride epitaxial layer. The substrate is taken out from a MOCVDchamber. A (SiO₂)₂₃ mask having 1-120 stripes partially overlaps thebase layer 22. Then, HVPE or MOCVD is lo performed to grow epitaxially ahigh-temperature gallium nitride epitaxial layer 24. With the use of the(SiO₂)₂₃ mask, the epitaxial growing mechanism is selectively performed.The growth direction of the epitaxial layer vertical to that of thegallium nitride in the areas of the base layer not covered by the(SiO₂)₂₃ mask. After the formed epitaxial layer reaches the same levelas the mask, the layer is continuously grown, faster than before, in thegrowth direction of the gallium nitride base layer, thereby preventingdefects from spreading in the vertical direction. The defect density ofthe subsequently epitaxially grown gallium nitride layer is thereforereduced. However, the defect density is not reduced until the layer ishas a thickness more than 10 μm. This kind of lateral epitaxial growtheffectively reduces the defect density, but complicates the productionof the mask. Meanwhile, the elective growth mechanism increases thewhole production cost.

U.S. Pat. No. 6,475,882 discloses a lateral epitaxial growth using a SiNmicro-mask. Before an epitaxial process is conducted, precursorsincluding SiH₄ and NH₃ form SiN islands on a sapphire-based substrate.The SiN islands are used as a mask for subsequent lateral epitaxialgrowth to reduce the defect density. According to this disclosure, theflow and the reaction time of the precursors are controlled so as toobtain an epitaxial film with good crystallization. However, theuniformity and the density of the SiN micro-mask are not easilycontrolled, and the yield is not easily controlled, either.

Therefore, there is a need for a process of forming an epitaxial layer,suitable for the formation of an epitaxial layer of an illuminationdevice, the epitaxial layer having perfect crystallization and littledislocation, low (Dst and improved yield.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an epitaxialstructure of a gallium nitride series semiconductor and a process ofmanufacturing the same. A first buffer layer made of gallium nitride isepitaxially grown over a substrate at a first temperature. A secondbuffer layer made of indium gallium nitride is formed on the firstbuffer layer at a second temperature. The second temperature is thenincreased to a third temperature. While the temperature is increasing,precursors such as In(CH₃)₃ and NH₃ are used for surface treatment ofthe second buffer layer. Thereafter, an epitaxial gallium nitride layeris grown at the third temperature. The structure of the obtained bufferlayer and the process of growing such a buffer layer allow perfectcrystal and low defect density, effectively increasing the performanceand service life of a semiconductor device.

To provide a further understanding of the invention, the followingdetailed description illustrates embodiments and examples of theinvention, this detailed description being provided only forillustration of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will be more readily appreciated as the same becomes betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a conventional gallium nitride structuregrown with the use of a buffer layer;

FIG. 2 is a schematic view of another conventional gallium nitridestructure grown with the use of a buffer layer;

FIG. 3 is a front view of an epitaxial structure of a gallium nitrideseries semiconductor device according to one embodiment of theinvention;

FIG. 4 is a flowchart of an epitaxial structure of a gallium nitrideseries semiconductor device according to one embodiment of theinvention;

FIG. 5 is a flowchart of an epitaxial structure of a gallium nitrideseries semiconductor device according to another embodiment of theinvention;

FIG. 6 is a FWHM graph of an indium gallium nitride layer obtained bythe invention; and

FIG. 7 is a phase difference of an epitaxial structure of an indiumgallium nitride layer according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Wherever possible in the following description, like reference numeralswill refer to like elements and parts unless otherwise illustrated.

In one preferred embodiment of the invention, the invention provides anepitaxial structure of a gallium nitride series semiconductor device.Referring to FIG. 3, an epitaxial structure 30 includes a substrate 31,a first gallium nitride buffer layer 32, a second indium gallium nitridebuffer layer 33 and an epitaxial gallium nitride layer 34. In theepitaxial structure 30, the substrate 31 is formed from sapphire, SiC,ZnO or Si. The first, gallium nitride buffer layer 32 further includes,from bottom to top, a low-temperature gallium nitride buffer layer 32′and a high-temperature gallium nitride buffer layer 32″. Furthermore,the epitaxial structure 30 further includes an epitaxy gallium nitridelayer 35 on the epitaxial gallium nitride 34.

FIG. 4 is a flowchart illustrating the formation of the epitaxialstructure according to one embodiment of the invention. A substrate 31is provided, and a thermal cleaning is performed on the substrate 31(step 41). The low-temperature gallium nitride buffer layer 32′ isformed on the substrate 31 at the first temperature (step 42). Increasedtemperature allows the formation of crystal cores over the first galliumnitride buffer layer to form high-temperature gallium nitride bufferlayer 32″ (step 43). The temperature is decreased to a secondtemperature at which the second buffer layer 33 is formed on thehigh-temperature gallium nitride buffer layer 32″ (step 44). Thetemperature increases to a third temperature at which the epitaxialgallium nitride layer 34 is formed on the second buffer layer 33 (step45). The temperature relationship is as follows: the firsttemperature<the second temperature<the third temperature.

FIG. 5 is a flowchart illustrating the formation of an epitaxialstructure according to another embodiment of the invention. Thesubstrate 31 is provided and a surface cleaning is performed on thesubstrate 31 (step 51). The 15 low-temperature gallium nitride bufferlayer 32″ is formed on the substrate 31 at the first temperature (step52). As the temperature further increases, crystal cores are formed onthe first buffer layer to form a high-temperature gallium nitride bufferlayer 32″ (step 53). The indium gallium nitride buffer layer 33 isformed on the high-temperature gallium nitride buffer layer 32″ when thethird 20 temperature is decreased to the second temperature (step 54).The second temperature is again increased to the third temperature.During this period, the precursors including In(CH₃)₃ and NH₃ are usedto perform a surface treatment of the indium gallium nitride bufferlayer 33 (step 55). The high-temperature epitaxial gallium nitride layer34 is formed at the third temperature (step 56). The temperaturerelationship is as follows: the first temperature<the secondtemperature<the third temperature.

When the epitaxial structure 30 is to be formed, the temperature (thefirst temperature) at which the first gallium nitride buffer layer 32 isformed is 400° C. to 800° C. The first buffer layer 32 has a thicknessof 200 nm to 40 nm, and has a multiple crystal structure epitaxiallygrown by metal organic chemical vapor deposition (MOCVD). Thetemperature (the second temperature) at which the second buffer layer 33is formed is 830° C. to 880° C. The second buffer layer 33 has athickness of 40 nm to 60 nm, and has a single crystal structureepitaxially grown by metal organic chemical vapor deposition (MOCVD).FIG. 6 is a FWHM graph of an indium gallium nitride layer. FIG. 7 is aphase difference of an indium gallium nitride layer. The epitaxialgallium nitride 34 has a single crystal structure, with a defect densitylower than 1×10⁸ cm ². The difference between the temperature at whichthe low-temperature gallium nitride buffer layer 32′ is formed and thetemperature at which the high-temperature gallium nitride buffer layer32″ is formed must be more than 300° C. Furthermore, the material usedto form the epitaxial gallium nitride series layer is one selected fromthe group consisting of the following componentsB_(x)Al_(y)In_(z)Ga_(1-x-y-z)N_(p)As_(q) (0≦x≦1, 0<y≦1, 0≦z≦1, 0≦p≦1,0≦q≦1 and x+y+z=1, p+q=1).

It should be apparent to those skilled in the art that the abovedescription is only illustrative of specific embodiments and examples ofthe invention. The invention should therefore cover variousmodifications and variations made to the herein-described structure andoperations of the invention, provided they fall within the scope of theinvention as defined in the following appended claims.

1-11. (canceled)
 12. A process of forming an epitaxial structure of agallium nitride series semiconductor, comprising: providing a substrate;forming a first buffer layer of gallium nitride on the substrate at afirst temperature; forming a second buffer layer of indium galliumnitride on the first buffer layer; increasing temperature up to a thirdtemperature, during which precursors including In(CH3)3 and NH3 are usedfor surface treatment; and growing a high-temperature an epitaxial layerof gallium nitride at the third temperature; wherein the firsttemperature is less than the second temperature, and the secondtemperature is less than the third temperature.
 13. The process of claim12, wherein the first temperature is about 400° C. to 800° C.
 14. Theprocess of claim 12, wherein the second temperature is about 800° C. to830° C.
 15. The process of claim 12, further comprising growing agallium nitride series epitaxial layer on the gallium nitride epitaxiallayer.
 16. The process of claim 12, wherein the step of forming thefirst buffer layer at the first temperature includes forming alow-temperature gallium nitride buffer layer on the substrate at thefirst temperature, and increasing the first temperature to form crystalcores on the low-temperature gallium nitride buffer layer, therebyforming a high-temperature gallium nitride buffer layer.
 17. The processof claim 16, wherein a difference between the first temperature and atemperature for forming the high-temperature gallium nitride bufferlayer is more than 300° C.